Direct current (DC) power transmission is used in a number of different applications. High-voltage DC (HVDC) is particularly useful for power transmission over long distances and/or interconnecting alternating current (AC) networks that operate at different frequencies. A first station may therefore transmit electrical energy to a second station over a DC transmission line, e.g. an overhead line or subsea or buried cable. The first station may generate the DC supply by conversion from a received AC input supply. The second station then typically provides conversion back from DC to AC. Each of the first and second stations may therefore typically comprise a voltage source converter (VSC) (or any other suitable mechanism, such as a line-commutated converter for example) for converting from AC to DC or vice versa. More complicated networks comprising multiple DC links and multiple stations are also known.
Cables using extruded insulating materials, such as the polyolefin group and its composite counterparts, are well known. When used in AC applications, where the voltage varies cyclically about ground at the grid frequency (e.g. 50 Hz or 60 Hz), space charge has no opportunity to accumulate in the cable as the polarity of the electric field constantly varies about zero.
In DC applications, however, the polarity of the electric field is constant and this can lead to space charge accumulation in the cable via a number of different mechanisms. In HVDC systems, where conditions include voltages in the order of hundreds of kilovolts, space charge can accumulate even in the low mobility carriers which are inherent to extruded cable insulation.
Space charge accumulation in extruded cables accelerates cable ageing and can lead to premature insulation breakdown or reduced rating. Methods and devices for mitigating space charge accumulation in HVDC systems are therefore required.